Formed member and method for manufacturing same

Abstract

The present invention relates to a formed member used in parts such as automotive structural members and reinforcements, and to a method for manufacturing same.

Description

Molded member and manufacturing method thereof

[0001] The present invention relates to a molded member used in parts such as automobile structural members and reinforcing members, and a method for manufacturing the same.

[0002] With the increasing need for passenger safety regulations and weight reduction in automobiles, research is being conducted on increasing the strength of steel plates to improve the crashworthiness of vehicle bodies. However, increasing the strength of automotive steel plates significantly reduces their formability, limiting their potential for high strength.

[0003] A representative technology for resolving this issue is Patent Document 1, which discloses a steel sheet with excellent formability and a tensile strength of 800 MPa utilizing martensitic transformation of retained austenite. However, to secure a strength higher than 1000 MPa, large amounts of elements such as C and Mn must be added, which increases manufacturing costs and causes problems such as die wear for cold forming and difficulty in shape correction.

[0004] Patent documents 2 and 3 are technologies that can secure higher strength while solving such problems. These patents disclose ultra-high strength steel sheets that utilize low strength and high workability before heat treatment to perform heat treatment and press forming in the austenite single-phase region, and then rapidly cool using a mold to form a structure close to the martensite single-phase in the final product.

[0005] However, the above technologies have limitations in the automotive parts they can be applied to, as they have difficulty in securing sufficient crashworthiness properties due to high hardness and low elongation.

[0006] (Patent Document 1) Korean Patent Publication No. 2007-0110914

[0007] (Patent Document 2) Korean Patent Publication No. 2007-0057689

[0008] (Patent Document 3) U.S. Patent No. 6,296,805

[0009] One aspect of the present invention relates to a molded member having excellent crash resistance characteristics by simultaneously securing high strength and excellent ductility through heat treatment, and a method for manufacturing the same.

[0010] The objectives of the present invention are not limited to the above-described matters. Additional objectives of the present invention are described throughout the specification, and those skilled in the art will have no difficulty understanding the additional objectives of the present invention from the contents described in the specification.

[0011] One embodiment of the present invention comprises, in wt%, C: 0.1 to 1.0%, Si+Al: 0.4 to 3.0%, Mn: 0.1 to 5.0%, P: 0.0001 to 0.1%, S: 0.0001 to 0.03%, the remainder being Fe and other unavoidable impurities.

[0012] The microstructure contains 1 to 10% area fraction of retained austenite, 80 to 99% area fraction of martensite, and 10% or less (including 0%) of ferrite.

[0013] It relates to a molded member in which P_El / U_El, as defined by the following equation 1, is 1.0 or more.

[0014] [Formula 1]

[0015] P_El / U_El ={(T_El)-(U_El)} / (U_El)

[0016] In the above equation 1, U_El represents uniform elongation, and T_El represents total elongation.

[0017] The above-mentioned molded member may further include one or more of the following a) to f).

[0018] a) One or more selected from the group consisting of Cr, Mo and W: 0.01 to 2.0%,

[0019] b) One or more elements selected from the group consisting of Ti, Nb, Zr and V: 0.001 to 0.4%,

[0020] c) 0.005 to 2.0% of one or both of Cu and Ni,

[0021] d) B: 0.0001~0.01%,

[0022] e) Sb: 0.001~0.1%, and

[0023] f) As: 1.00% or less

[0024] The above-mentioned molded member may have a yield ratio (YR=YS / TS) value of 0.85 or more.

[0025] The above-mentioned molded member may have a hole expansion ratio (HER) of 30% or more.

[0026] The above-mentioned molded member may have a tensile strength of 1500 MPa or more and a yield strength of 1300 MPa or more.

[0027] Another embodiment of the present invention is a forming step of forming a steel material comprising, in wt%, C: 0.1 to 1.0%, Si+Al: 0.4 to 3.0%, Mn: 0.1 to 5.0%, P: 0.0001 to 0.1%, S: 0.0001 to 0.03%, the remainder Fe, and other unavoidable impurities;

[0028] A rapid cooling step of rapidly cooling the formed steel plate to a temperature range of 0.3*Mf or more to less than Mf; and

[0029] The present invention relates to a method for manufacturing a molded member, including a heating step of reheating the rapidly cooled steel plate to a temperature range below Ac1 and then maintaining the temperature.

[0030] The above steel may further include one or more of the following a) to f).

[0031] a) One or more selected from the group consisting of Cr, Mo and W: 0.01 to 2.0%,

[0032] b) One or more elements selected from the group consisting of Ti, Nb, Zr and V: 0.001 to 0.4%,

[0033] c) 0.005 to 2.0% of one or both of Cu and Ni,

[0034] d) B: 0.0001~0.01%,

[0035] e) Sb: 0.001~0.1%, and

[0036] f) As: 1.00% or less

[0037] Before the above forming step, a heating step of heating the steel to an austenite region higher than Ac3 temperature may be further included.

[0038] After the above forming step, a heating step of heating to an austenite region higher than Ac3 temperature may be further included.

[0039] The above rapid cooling can be performed at a speed of 10 to 500°C / s.

[0040] After the above reheating, the holding time can be 1 to 10,000 seconds.

[0041] According to one aspect of the present invention, a molded member having ultra-high strength and ductility that can be used for automobile structural members and reinforcing materials can be provided, and further, the application can be expanded to heat-treated crash members.

[0042] The various advantageous and beneficial advantages and effects of the present invention are not limited to the above-described contents, and will be more easily understood in the course of explaining specific embodiments of the present invention.

[0043] The terminology used herein is for the purpose of describing the present invention and is not intended to limit the present invention. Furthermore, the singular forms used herein also include the plural forms, unless the context clearly dictates otherwise.

[0044] The meaning of "comprising" as used in the specification is to specify a configuration and not to exclude the presence or addition of other configurations.

[0045] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in the dictionary are to be interpreted to have meanings consistent with the relevant technical literature and the present disclosure.

[0046] Preferred embodiments of the present invention are described below. These embodiments may be modified in various ways, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to further illustrate the present invention to those skilled in the art.

[0047] The present invention aims to improve the elongation of a formed part by appropriately controlling the composition when manufacturing a formed part, rapidly cooling the steel plate to a temperature range of 0.3*Mf to less than Mf, and then reheating and maintaining it to a temperature lower than Ac1, thereby improving the elongation of the formed part. That is, while a conventional method for manufacturing a formed part using a heat-treatable steel involves hot-forming a steel plate and then rapidly cooling it to manufacture a high-strength formed part with martensite as the main phase, the present invention utilizes the fact that when cooling to a temperature range of 0.3*Mf to Mf to form retained austenite and then reheating to appropriately maintain the retained austenite, high strength and ductility can be secured at the same time.

[0048] A molded member refers to a processed product formed by processing a steel plate into a certain shape, and includes all members that can be applied to a purpose that can utilize the physical properties of the steel of the present invention.

[0049] Hereinafter, Ac1, Ac3, Ms and Mf in the present invention are described as follows. Here, each element means the content (weight %).

[0050] Ac1 is the temperature at which austenite begins to form when the steel plate is heated and the temperature is increased, and is defined as follows:

[0051] Ac1(℃) = 723-10.7Mn-16.9Ni+29.1Si+16.9Cr+290As+6.38W

[0052] Ac3 is the temperature at which austenite transformation is completed when the steel plate is heated and the temperature is increased, and is defined as follows.

[0053] Ac3(℃) = 910-203√C-15.2Ni+44.7Si+104V+31.5Mo+13.1W

[0054] Ms is the temperature at which martensite begins to form when a steel plate heated to austenite is cooled, and is defined as follows:

[0055] Ms(℃) = 548-440C-14Si-26Mn-11Cr-14Ni-9Mo+2V

[0056] Mf is the temperature at which martensite transformation is completed and is defined as follows:

[0057] Mf(℃) = Ms - 150℃

[0058] A molded member, which is an example of the present invention, is described.

[0059] First, the composition is described in detail. Unless otherwise specified, the % and ppm stated in relation to the alloy composition are based on weight.

[0060] The above molded member may contain, in wt%, C: 0.1 to 1.0%, Si+Al: 0.4 to 3.0%, Mn: 0.1 to 5.0%, P: 0.0001 to 0.1%, S: 0.0001 to 0.03%, the remainder Fe, and other unavoidable impurities.

[0061] In addition, it may further include at least one selected from the group consisting of Cr, Mo, and W at 0.01 to 2.0%, at least one selected from the group consisting of Ti, Nb, Zr, and V at 0.001 to 0.4%, one or two selected from Cu or Ni at 0.005 to 2.0%, and one or two selected from B: 0.0001 to 0.01% or Sb: 0.001 to 0.1%.

[0062] Carbon (C): 0.1~1.0% (% below means weight%)

[0063] C is not only an essential element for increasing the strength of steel plates, but also needs to be appropriately added to secure the retained austenite desired in the present invention. If the C content is less than 0.1%, sufficient strength cannot be obtained even if heat treatment is performed in the austenite single-phase region, and it is also difficult to secure more than 5% of retained austenite when heat treating a member after hot forming or cold forming. In addition, if the C content exceeds 1.0%, the toughness and weldability are likely to deteriorate, and not only does it make welding of the steel plate difficult during the pickling and rolling processes of the hot-rolled steel plate during the manufacturing process, but also the strength of the steel plate is too high during the annealing and plating processes, making cold forming difficult. Accordingly, the C content range is preferably set to be 0.1 to 1.0%.

[0064] Silicon (Si) + Aluminum (Al): 0.4~3.0%

[0065] Si and Al play a very important role in the present invention. Si and Al are elements that prevent carbon from precipitating in martensite when the steel sheet is rapidly cooled to between Ms and Mf and then maintained below the Ac1 temperature, thereby concentrating a large amount of carbon into untransformed retained austenite and securing stable retained austenite in the final member. If the total content of Si and Al is less than 0.4%, it is difficult to expect this effect, and if it exceeds 3%, it is difficult to remove surface scale of the hot-rolled steel sheet and the heat treatment temperature for manufacturing the member is increased, which causes an increase in manufacturing cost. Therefore, the content is preferably set to 0.4 to 3.0%.

[0066] Manganese (Mn): 0.1~5.0%

[0067] Manganese (Mn) is a solid solution strengthening element that not only contributes to increasing strength, but also delays the transformation from austenite to ferrite and lowers the Ac3 temperature. When the Mn content is less than 0.1%, a high heat treatment temperature is required to heat treat the steel sheet in the austenite single-phase region, which accelerates oxidation of the steel sheet and deteriorates the heat resistance of the coated steel sheet even if it is used. In addition, the desired high strength cannot be secured by heat treatment in the two-phase region where ferrite and austenite coexist. When the Mn content exceeds 5.0%, problems such as weldability and hot-rollability arise, so the Mn content range is preferably 0.1 to 5.0%.

[0068] Phosphorus (P): 0.0001~0.1%

[0069] Similar to Si, P exhibits an effect of suppressing carbide formation during martensite heat treatment. However, excessive P content deteriorates weldability, so the upper limit is limited to 0.1%. However, controlling P to less than 0.0001% incurs significant manufacturing costs, so it is desirable to limit the lower limit to 0.0001%.

[0070] Sulfur (S): 0.0001~0.03%,

[0071] Sulfur (S) exists as an impurity in steel and is an element that impairs the ductility and weldability of steel plates. Since these adverse effects are minimal at S contents below 0.03%, it is desirable to set the upper limit at 0.03%. However, controlling the S content below 0.0001% incurs significant manufacturing costs, so it is desirable to set the lower limit at 0.0001%.

[0072] In addition to the steel composed as described above, at least one selected from the group consisting of Cr, Mo, and W, which are hardenability enhancing elements, may be additionally included in an amount of 0.01 to 2.0%, at least one selected from the group consisting of Ti, Nb, Zr, and V, which are precipitation strengthening elements, may be additionally included in an amount of 0.001 to 0.4%, one or two selected from Cu or Ni, which are strength enhancing elements, may be additionally included in an amount of 0.005 to 2.0%, and B: 0.0001 to 0.01% as a grain boundary strengthening and hardenability element or Sb: 0.001 to 0.1% for improving plating properties may be additionally included.

[0073] At least one of chromium (Cr), molybdenum (Mo), and tungsten (W): 0.01 to 2.0%

[0074] Cr, Mo, and W enhance hardenability, which significantly contributes to achieving high strength. Furthermore, because they enhance hardenability, they can maintain adequate strength even when cooling performance is compromised due to incomplete contact with the mold during high-temperature forming. When the Cr, Mo, or W content is below 0.01%, sufficient hardenability cannot be achieved, and when it exceeds 2.0%, the effect becomes saturated and manufacturing costs increase. Therefore, a content range of 0.01 to 2.0% is recommended.

[0075] At least one of titanium (Ti), niobium (Nb), zirconium (Zr), and vanadium (V): 0.001 to 0.4%

[0076] Ti, Nb, Zr, and V are elements that enhance the strength, grain refinement, and heat treatment properties of steel sheets. If the content of Ti, Nb, Zr, and V is less than 0.001%, it is difficult to expect the above-mentioned effects, and if the content exceeds 0.4%, excessive manufacturing costs will increase. Therefore, it is preferable to keep the content between 0.001 and 0.4%.

[0077] At least one of copper (Cu) and nickel (Ni): 0.005–2.0%

[0078] Copper (Cu) is an element that enhances strength by forming fine Cu precipitates. If the Cu content is less than 0.005%, the desired strength cannot be sufficiently achieved, and if it exceeds 2.0%, workability may be impaired. Meanwhile, Ni (Ni) is an effective element for increasing strength and improving heat-treatability. However, if the Cu content is less than 0.005%, the effect cannot be achieved, and if it exceeds 2.0%, manufacturing costs increase. Therefore, the Cu and Ni contents are preferably set at 0.005 to 2.0%.

[0079] Boron (B): 0.0001~0.01%

[0080] B is an element with high hardenability, and even a small amount can secure high strength in heat-treated steel. However, if it is less than 0.0001%, this effect cannot be achieved, and if it exceeds 0.01%, not only is the effect saturated, but hot workability deteriorates. Therefore, it is preferable to keep the content between 0.0001 and 0.01%.

[0081] Antimony (Sb): 0.001~0.1%

[0082] Sb is a surface-enriching element, and can suppress the formation of oxides due to surface enrichment of Si and Al added in the present invention during annealing, which deteriorates plating properties. However, this effect cannot be achieved when the content is less than 0.001%, and when it exceeds 0.1%, hot workability deteriorates. Therefore, it is preferable to set the content to 0.001 to 0.1%.

[0083] Arsenic (As): 1.00% or less

[0084] As may be additionally included considering the target properties of the final product, etc., and if its content exceeds 1.00%, it significantly increases manufacturing costs, so its content may be limited.

[0085] In addition to the aforementioned components, the remaining iron and other unavoidable impurities may be included. However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in during the normal manufacturing process, they cannot be completely excluded. Since these impurities are readily apparent to anyone skilled in the art, their full contents are not specifically mentioned in this specification.

[0086] In addition, the addition of additional effective ingredients other than the aforementioned ingredients is not completely ruled out.

[0087] The microstructure of the above-mentioned molded member is preferably composed of, in area fraction, 1 to 10% of retained austenite, 80 to 99% of martensite, and 10% or less (including 0%) of ferrite.

[0088] When the residual austenite fraction is less than 1%, it is difficult to secure the high elongation targeted by the present invention. However, to secure this residual austenite fraction exceeding 10%, excessive addition of C, Si, and Al is required, which poses a problem of manufacturing difficulty.

[0089] It is preferable that the structure other than the above-mentioned retained austenite be composed of martensite. It is preferable that the martensite fraction be 80-99%. If the martensite fraction is less than 80%, it is difficult to sufficiently secure the desired strength, and if it exceeds 99%, sufficient retained austenite cannot be secured.

[0090] However, ferrite may be formed during slow cooling after heat treatment at temperatures above Ac3, and it is desirable to limit the upper limit to 10%. If the ferrite fraction exceeds 10%, it is difficult to sufficiently secure the desired strength.

[0091] It is preferable that the molded member of the present invention have a yield ratio (YS / TS) of 0.85 or more. If the yield ratio is less than 0.85, the yield strength is low at similar tensile strengths, which is not sufficient to ensure crashworthiness.

[0092] It is preferable that the above-mentioned molded member has a P_El / U_El of 1.0 or more. The P_El / U_El is defined by the following equation 1.

[0093] [Formula 1]

[0094] P_El / U_El ={(T_EL)-(U_EL)} / (U_El)

[0095] In the above equation 1, U_El represents uniform elongation, and T_El represents total elongation.

[0096] The above U_El (Uniform Elongation) and P_El (Peak Elongation) values ​​can be calculated based on the KS B 0802:1998 standard. U_El represents the elongation at maximum tensile load, and P_El represents the value obtained by subtracting the uniform elongation (U_El) from the total elongation (T_El).

[0097] The above P_El / U_El value being 1.0 or greater means that the P_El value is greater than the U_El value, which means that even after necking begins, there is a high resistance to coalescence and propagation of voids, and this can be understood to have a correlation with a high HER (Hole Expansion Ratio) value. Therefore, when the P_El / U_El value is less than 1.0, it is difficult to secure a sufficient HER value, which can lead to a problem of not being able to secure crashworthiness, so it is desirable to set the lower limit to 1.0.

[0098] It is preferable that the above-mentioned molded member have a HER value of 30% or more. The HER test can be measured according to ISO 16630:2009. If the HER value is less than 30%, it is a value indicating that it is difficult to suppress the growth of micro voids existing within the material. Through various experiments, it was discovered that if the value is less than 30%, cracks can easily be created and propagated when the member folds during a collision, and this causes a problem in that it is difficult to secure crashworthiness.

[0099] The above-mentioned molded member may have a tensile strength of 1500 MPa or more and a yield strength of 1300 MPa or more.

[0100] Below, an example of a method for manufacturing a molded member of the present invention is described in detail.

[0101] The above-mentioned method for manufacturing a molded member can be manufactured through processes of forming, rapidly cooling, and heat treating steel. Each process is described in detail below.

[0102] plastic surgery

[0103] First, press forming is performed using steel, either hot press forming or cold press forming followed by heat treatment. The hot press forming is performed by heating the steel sheet to an austenite region higher than Ac3 temperature before press forming, and then press forming, while cold press forming is performed by pressing the steel sheet manufactured above in the cold and then heating the steel sheet to an austenite region higher than Ac3 temperature.

[0104] The above steel material satisfies the aforementioned alloy composition and can be used in the technical field of the present invention, and there are no particular limitations on the type or manufacturing method. For example, the above steel material may be a steel plate, such as a hot-rolled steel plate, a cold-rolled steel plate, or a plated steel plate.

[0105] As an example of a method for manufacturing the above steel, a steel slab having the above composition is heated at 1000 to 1300°C, and then hot-rolled. If the heating temperature is less than 1000°C, the homogenization of the cast steel structure is not sufficiently achieved, and if it exceeds 1300°C, there is a high possibility that the manufacturing cost will increase. Thereafter, hot finishing rolling is completed at a temperature above the Ar3 temperature and below 1000°C. If the hot finishing rolling temperature is below the Ar3 temperature, abnormal rolling may occur, which may cause hot-rolled mixed grains and deteriorate the operability. If it exceeds 1000°C, it may cause grain coarsening. Then, coiling is performed at a temperature exceeding the Ms temperature and below 750°C. If it is below the Ms temperature, martensitic transformation occurs, which has the disadvantage of making the strength of the hot-rolled steel sheet excessively high, and if it exceeds 750°C, there is the disadvantage of increasing the thickness of the oxide layer of the hot-rolled steel sheet. The hot-rolled steel sheet manufactured as described above can be used directly as a heat-treated member or, after pickling, as a molded member. Furthermore, to improve corrosion resistance, the hot-rolled steel sheet or the pickled hot-rolled steel sheet can be plated, and such plated steel sheet is also suitable for use as a molded member. However, the type and method of the plating are not particularly limited, as they can utilize any conventional method in the relevant technical field.

[0106] The hot-rolled steel sheet manufactured as described above is subjected to pickling and cold rolling. There is no particular limitation on the reduction ratio during the cold rolling, and it can be performed under normal conditions. The cold-rolled steel sheet is subjected to continuous annealing or phase annealing, and the conditions of the annealing process are not particularly limited; however, it is preferable to lower the strength before heat treatment to facilitate cold forming and heat treatment. The cold-rolled steel sheet can also be plated like the hot-rolled steel sheet, but this also has a minimal effect on securing the material sought by the present invention, and therefore is not particularly limited.

[0107] As plating methods for the above-mentioned hot-rolled steel sheets and cold-rolled steel sheets, aluminum plating, zinc plating, alloy zinc plating, and zinc electroplating can be used. In addition, resin coating can be used instead of the above-mentioned plating, and the resin coating can be performed using one or two types of sol-gel or aluminum powder.

[0108] Meanwhile, before the above forming, the steel material can be processed into a blank shape and provided for effective forming.

[0109] Rapid cooling

[0110] The above-mentioned formed steel is rapidly cooled to a temperature range of 0.3*Mf or more to less than Mf.

[0111] Generally, it is known that retained austenite does not exist when the quenching temperature is below Mf. However, our technicians have devised a method to secure retained austenite even when the quenching temperature is below Mf. They have discovered that the retained austenite obtained at this temperature is finer and more stable than the retained austenite obtained when the quenching temperature exceeds Mf, thereby contributing to improved elongation.

[0112] When the quenching temperature is above Mf, the retained austenite fraction increases, and even if carbon is concentrated in the retained austenite after reheating, sufficient stability is difficult to ensure, resulting in a lower yield strength. When the quenching temperature is below 0.3 Mf, the retained austenite fraction is less than 1%, making it difficult to secure sufficient retained austenite to improve elongation.

[0113] At this time, it is preferable that the rapid cooling is performed at a speed of 10 to 500°C / s. If the rapid cooling speed is less than 10°C / s, there is a problem that a sufficient amount of martensite cannot be secured because ferrite or bainite structures may be generated instead of martensite due to the low cooling speed. If it exceeds 500°C / s, martensite can be sufficiently secured, but the manufacturing cost may increase due to the excessively high cooling speed.

[0114] heat treatment

[0115] Thereafter, the rapidly cooled steel plate can be heated to a temperature range lower than Ac1 and then maintained therein. If heat treatment is performed at this heating temperature, that is, the tempering temperature, in the ferrite and austenite ideal range exceeding Ac1, not only will a large amount of ferrite be generated, but softening of the generated martensite will rapidly progress, making it impossible to secure the desired high strength. In the present invention, there is no particular limitation on the lower limit of the tempering temperature, but if it is lower than the Ms temperature, it takes too much time for carbon to diffuse into the austenite remaining during rapid cooling, so it is preferably performed at the Ms temperature or higher.

[0116] At this time, after the heat treatment, it is preferable that the holding time be in the range of 1 to 10,000 seconds. If it is less than 1 second, the carbon in the martensite is not sufficient to concentrate into the remaining austenite, and if it exceeds 10,000 seconds, the martensite softening progresses excessively, making it difficult to secure the desired strength.

[0117] Hereinafter, embodiments of the present invention will be described. It should be understood that those skilled in the art will appreciate that various modifications to the following embodiments can be made without departing from the scope of the present invention. The following embodiments are intended to facilitate understanding of the present invention, and the scope of the present invention should not be limited to the following embodiments, but should be determined not only by the claims set forth below but also by their equivalents.

[0118] (Example)

[0119] A steel slab having the composition components (unit weight %, the remainder being Fe and unavoidable impurities) shown in Table 1 below was vacuum melted, heated in a furnace at 1000 to 1300°C for 1 hour, and then hot rolled. The hot-rolled steel sheet was cooled and then furnace-quenched at a predetermined temperature. At this time, the hot rolling was completed in the temperature range of 850 to 950°C, and the furnace-quenching temperature was 680°C. Thereafter, pickling was performed using the hot-rolled steel sheet, and rolling was performed with a cold reduction ratio of 50%. The cold-rolled steel sheet was annealed at 780°C, and then continuous annealing was performed at an overaging temperature of 400°C to complete the manufacture of a cold-rolled steel sheet.

[0120]

[0121] In order to simulate the heat treatment of the hot press forming process, as shown in Table 2, the manufactured steel plate was heated at 30℃ / sec, maintained at 900℃, which is higher than the Ac3 temperature, for 90 sec, and then cooled to a predetermined temperature (T1) and maintained for a predetermined time (t1). After maintaining for 2 sec, the plate was reheated to the predetermined heat treatment temperature (T2), maintained for a predetermined time (t2), and then cooled to room temperature at 50℃ / sec, thereby conducting a heat treatment simulation. At this time, the heating temperature and the T1 and T2 temperatures were obtained through a dilatation test. From this, the Ac3, Ac1, and Ms temperatures were obtained, and the Mf temperature at which 90% transformation was obtained using the lever rule. The mechanical properties of the steel plate obtained from this were measured using a JIS Z 2201 No. 5 tensile test specimen, such as yield strength (YS), tensile strength (TS), uniform elongation (U_El), and total elongation (T_El). The retained austenite fraction was calculated using the 5-peak method as in Equation 2 below by obtaining the areas of the austenite (200), (220), (311) peaks and the ferrite (200), (211) peaks obtained from the X-ray diffraction test. The mechanical properties and retained austenite fraction of the final product obtained from this are shown in Table 3 below.

[0122] Meanwhile, in Table 3 below, P_El / U_El is calculated as in Equation 1 below.

[0123] [Formula 1]

[0124] P_El / U_El ={(T_El)-(U_El)} / (U_El)

[0125] [Formula 2]

[0126]

[0127] Meanwhile, HER (Hole Expansion Ratio) was calculated as shown in Equation 3 below based on the ISO 16630-2009 standard.

[0128] [Formula 3]

[0129] HER = (Dh-D0) / D0 x100

[0130] Here, Dh is the average hole diameter after the HER test, and D0 is the hole diameter before the test.

[0131]

[0132]

[0133] As can be seen from Tables 1 to 3 above, an invention example that satisfies the composition and manufacturing conditions proposed by the present invention can provide a molded member having not only high strength but also excellent ductility.

[0134] In contrast, when the invention deviates from its proposal, strength or ductility is not secured.

[0135] As described above, the detailed description of the present invention has described preferred embodiments of the present invention. However, it will be apparent to those skilled in the art that various modifications may be made without departing from the scope of the present invention. Therefore, the scope of the rights of the present invention should not be limited to the described embodiments, but should be determined not only by the claims described below but also by equivalents thereof.

Claims

1. Contains, by weight%, C: 0.1~1.0%, Si+Al: 0.4~3.0%, Mn: 0.1~5.0%, P: 0.0001~0.1%, S: 0.0001~0.03%, the remainder being Fe and other unavoidable impurities. The microstructure contains 1–10% area fraction of retained austenite, 80–99% area fraction of martensite, and 10% or less (including 0%) of ferrite. A molded member having P_El / U_El of 1.0 or greater, as defined by the following Equation 1. [Formula 1] P_El / U_El ={(T_El)-(U_El)} / (U_El) In the above equation 1, U_El represents the uniform elongation and T_El represents the total elongation.

2. In claim 1, The above-mentioned molded member is a molded member comprising at least one of a) to f). a) One or more elements selected from the group consisting of Cr, Mo and W: 0.01 to 2.0%; b) One or more elements selected from the group consisting of Ti, Nb, Zr and V: 0.001 to 0.4%; c) 0.005 to 2.0% of one or both of Cu and Ni, d) B: 0.0001~0.01%, e) Sb: 0.001~0.1%, and f) As: 1.00% or less 3. In claim 1, The above-mentioned molded member is a molded member having a yield ratio (YR=YS / TS) value of 0.85 or more.

4. In claim 1, The above-mentioned molded member is a molded member having a pore expansion ratio (HER) of 30% or more.

5. In claim 1, The above-mentioned molded member is a molded member having a tensile strength of 1500 MPa or more and a yield strength of 1300 MPa or more.

6. A forming step for forming a steel material composed of C: 0.1 to 1.0% by weight, Si+Al: 0.4 to 3.0%, Mn: 0.1 to 5.0%, P: 0.0001 to 0.1%, S: 0.0001 to 0.03%, the remainder being Fe and other unavoidable impurities; A rapid cooling step of rapidly cooling the formed steel plate to a temperature range of 0.3*Mf or more to less than Mf; and A method for manufacturing a molded member, comprising a heating step of reheating the rapidly cooled steel plate to a temperature range below Ac1 and then maintaining it.

7. In claim 6, A method for manufacturing a formed member comprising at least one of a) to f). a) One or more elements selected from the group consisting of Cr, Mo and W: 0.01 to 2.0%; b) One or more elements selected from the group consisting of Ti, Nb, Zr and V: 0.001 to 0.4%; c) 0.005 to 2.0% of one or both of Cu and Ni, d) B: 0.0001~0.01%, and e) Sb: 0.001~0.1%, and f) As: 1.00% or less 8. In claim 6, A method for manufacturing a formed member, further comprising a heating step of heating the steel to an austenite range of Ac3 temperature or higher prior to the forming step.

9. In claim 6, A method for manufacturing a formed member, comprising, after the forming step, further comprising a heating step of heating to an austenite region having an Ac3 temperature or higher.

10. In claim 6, A method for manufacturing a molded part, wherein the above rapid cooling is performed at a speed of 10 to 500°C / s.

11. In claim 6, A method for manufacturing a molded part having a holding time of 1 to 10,000 seconds after the above reheating.

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